Geophysical exploration

Making, processing, and interpreting measurements of the physical properties of the Earth with the objective of practical application of the findings. Most exploration geophysics is conducted to find commercial accumulations of oil, gas, or other minerals, but geophysical investigations are also employed with engineering objectives, in studies aimed at predicting the nature of the Earth for the foundations of roads, buildings, dams, tunnels, nuclear power plants, and other structures, and in the search for geothermal areas, water resources, archeological ruins, and so on.

Geophysical exploration is also called applied geophysics or geophysical prospecting. The physical properties and effects of subsurface rocks and minerals that can be measured at a distance include density, electrical conductivity, thermal conductivity, magnetism, radioactivity, elasticity, and other properties. Exploration geophysics is often divided into subsidiary fields according to the property being measured, such as magnetic, gravity, seismic, electrical, thermal, or radioactive properties.

Magnetic exploration

Rocks and ores containing magnetic minerals become magnetized by induction in the Earth's magnetic field so that their induced field adds to the Earth's field. Magnetic exploration involves mapping variations in the magnetic field to determine the location, size, and shape of such bodies. The magnetic susceptibility of sedimentary rock is generally orders of magnitude less than that of igneous or metamorphic rock. Consequently, the major magnetic anomalies observed in surveys of sedimentary basins usually result from the underlying basement rocks. Determining the depths of the tops of magnetic bodies is thus a way of estimating the thickness of the sediments. See also Geomagnetism; Magnetometer; Rock magnetism.

Except for magnetite and a very few other minerals, mineral ores are only slightly magnetic. However, they are often associated with bodies such as dikes that have magnetic expression so that magnetic anomalies may be associated with minerals empirically. For example, placer gold is often concentrated in stream channels where magnetite is also concentrated.

Gravity exploration

Gravity exploration is based on the law of universal gravitation: the gravitational force between two bodies varies in direct proportion to the product of their masses and in inverse proportion to the square of the distance between them. Because the Earth's density varies from one location to another, the force of gravity varies from place to place. Gravity exploration is concerned with measuring these variations to deduce something about rock masses in the immediate vicinity. Gravity surveys are used more extensively for petroleum exploration than for metallic mineral prospecting. The size of ore bodies is generally small; therefore, the gravity effects are quite small and local despite the fact that there may be large density differences between the ore and its surroundings. See also Gravity meter; Prospecting.

Seismic exploration

Seismic exploration is the predominant geophysical activity. Seismic waves are generated by one of several types of energy sources and detected by arrays of sensitive devices called geophones or hydrophones. The most common measurement made is of the travel times of seismic waves, although attention is being directed increasingly to the amplitude of seismic waves or changes in their frequency content or wave shape. See also Seismology.

Electrical and electromagnetic exploration

Variations in the conductivity or capacitance of rocks form the basis of a variety of electrical and electromagnetic exploration methods, which are used primarily in metallic mineral prospecting. Both natural and induced electrical currents are measured. Direct currents and low-frequency alternating currents are measured in ground surveys, and ground and airborne electromagnetic surveys involving the lower radio frequencies are made. See also Geoelectricity.

Radioactivity exploration

Natural radiation from the Earth, especially of gamma rays, is measured both in land surveys and airborne surveys. Natural types of radiation are usually absorbed by a few feet of soil cover, so that the observation is often of diffuse equilibrium radiation. The principal radioactive elements are uranium, thorium, and potassium; radioactive exploration has been used primarily in the search for uranium and other ores, such as columbium, which are often associated with them. The Geiger counter and scintillation counter are instruments generally used to detect and measure the radiation. See also Geiger-Müller counter; Scintillation counter.

Remote sensing

Measurements of natural and induced electromagnetic radiation made from high-flying aircraft and earth satellites are referred to collectively as remote sensing. This comprises both the observation of natural radiation in various spectral bands, including both visible and infrared radiation, such as by photography and measurements of the reflectivity of infrared and radar radiation. See also Remote sensing.

Well logging

A variety of types of geophysical measurements are made in boreholes, including self-potential, electrical conductivity, velocity of seismic waves, natural and induced radioactivity, and temperature variations. Borehole logging is used extensively in petroleum exploration to determine the characteristics of the rocks which the borehole has penetrated, and to a lesser extent in mineral exploration. See also Well logging.

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Geophysics is a hybrid science (a combination of Geology and Physics) that achieved a distinctive identity only in the mid-twentieth century, which is understandable when it is recalled that neither geology nor physics emerged as distinctive disciplines until the mid-nineteenth century. The antecedents of geophysics reach back to Isaac Newton.

The geophysical exploration of the Americas began with the French expedition of 1735–1745 to the Peruvian Andes, which was led by Charles Marie de La Condamine and included Pierre Bouguer. Paired with a simultaneous venture to northern Scandinavia, the expedition attempted to verify the Newtonian prediction that the earth would be found to bulge at the equator and narrow at the poles, and the expeditions thus inaugurated the study of geodesy (measuring the earth's surface). About sixty years later, between 1799 and 1804, Alexander von Humboldt, accompanied by Aimé Bonpland, explored the American equatorial zone. In a romantic survey of natural history, Humboldt included such broadly geophysical measurements as terrestrial magnetism, which he linked to other physical phenomena—usually meteorological. These ventures set the style of geophysical exploration in America, establishing precedents for geophysics both as the direct object of an expedition—as in the pursuit of geodesy—and as a component of a larger reconnaissance including Cartography, specimen collection, and geophysical measurements. For nineteenth-century America, the broader Humboldtean model was the more powerful: geophysics was rather an instrumental component of geographic surveys than a conceptual framework for geologic interpretation. After Darwin, this pattern was incorporated into an evolutionary model that supplied the theoretical context of earth science for more than a century.

Concern with applying physical theories and techniques gradually created an informal alliance between three scientific groups. Planetary Astronomy, particularly as practiced by George H. Darwin, Osmond Fisher, and William Thomson (all British), addressed such problems as the formation, age, and structure of the earth. At the same time, "dynamical" geology, in contrast to historical or evolutionary geology, attempted to explain earth processes in terms of mechanics and physical laws. This group received some inspiration from such meteorologists as James Croll, who fashioned explanations for atmospheric dynamics based on physical processes; what physical chemistry and physical astronomy were to their respective disciplines, dynamical geology was to earth science. Finally, there was mining engineering, which provided technical training in metallurgy, mathematics, and physics at such schools as Columbia University and the University of California. Many of the instruments typical of geophysics, and many of the explorations that used them, stemmed from high-level prospecting, especially for oil.

In the exploration of the American West, geophysics is better understood in terms of certain themes and personalities than as a disciplinary science. Grove Karl Gilbert extended mechanics to problems in geomorphology and structural geology, framing quantitative geologic observations into rational systems of natural laws organized on the principle of dynamic equilibrium. Clarence E. Dutton, elaborating on speculations by John H. Pratt and George B. Airy, conceived the idea of isostasy, or the gravitational equilibrium of the earth's crust, and demonstrated how this pattern of vertical adjustment could become a compressive orogenic force (mountain formation, especially by the folding of the earth's crust). Dutton later made original contributions in volcanology and seismology. Samuel F. Emmons, Clarence King, and George F. Becker applied geochemical and geophysical analysis to the problems of orogeny and igneous ore formation. The latter two men were instrumental in establishing a chemistry laboratory in the U. S. Geological Survey and in applying its experimental results to geophysical phenomena. Becker explicitly attempted mathematical and mechanical models to describe ore genesis and the distribution of stress in the earth's crust. He was instrumental in the establishment of the Carnegie Institution's Geophysical Laboratory, served as the laboratory's first director, and bequeathed part of his estate to the Smithsonian Institution for geophysical research. Geophysics advanced in the context of a symbiosis (mutually beneficial relationship) of field exploration and laboratory investigation. In the case of the U. S. Geological Survey, Carl Barus staffed the laboratory and Robert S. Woodward furnished field geologists with information on mathematical physics. Woodward later directed the Carnegie Institution.

Following the work of the explorers, other geologists and geodesists—among them, Bailey Willis, Joseph Barrell, and J. F. Hayford—generated quantitative models for earth structure and tectonics. But explanations developed for glacial epochs best epitomized the status of geophysics: attempts to relate glacial movements to astrophysical cycles provided a common ground for geology, geophysics, astronomy, and Meteorology, but the results were rarely integrated successfully. Significantly, the most celebrated attempt at global geophysical explanation remained an unassimilated hybrid. In developing the planetesimal hypothesis in 1904, Thomas C. Chamberlin preserved a naturalistic understanding of earth geology, while F. R. Moulton supplied the mathematical physics. The earth sciences continued to subordinate their data and techniques to a broad evolutionary framework.

By the early twentieth century, geophysics was a conglomerate of pursuits promoted through federal scientific bureaus (Coast and Geodetic Survey, Geological Survey), private or university research institutes (Carnegie Institution's Geophysical Laboratory, the Smithsonian Institution), companies engaged in mineral prospecting, and exceptional individuals. Geophysics, having neither a disciplinary organization nor a unifying theory, remained more an analytic tool than a synthetic science.

This condition persisted until after World War II. Thereafter, with new instruments and techniques developed for mining and military purposes, with additional subjects (especially Oceanography), and with a theoretical topic to organize its research (continental drift), geophysics developed both an identity and a distinctive exploring tradition. This was well exemplified by the International Geophysical Year (IGY), planned for 1957 and 1958 but extended to 1959. In counterpoint to the space program, geophysicists proposed to drill into the interior of the earth. Although aborted in 1963, Project Mohole was superseded by other oceanic drill projects, especially the Joint Oceanographic Institute's Deep Earth Sampling Program (JOIDES) begun in 1964. The International Upper Mantle Project (1968–1972) formed a bridge between IGY and research under the multinational Geodynamics Project (1974–1979), which proposed to discover the force behind crustal movements.

In the late 1990s, geophysical exploration led to the Ocean Drilling Program. The program functioned from a ship called the JOIDES Resolution, named for the Joint Oceanographic Institutions for Deep Earth Sampling. Built to drill into the seabed for oil, the ship's high-technology laboratory was used by an international crew of scientists to conduct geophysical research. The program found evidence related to the impact of a meteorite at the end of the Cretaceous Era, evidence of ocean temperature changes in the Ice Ages, and documented changes in the earth's magnetic poles.

Geophysics is a revolution in physics, and its integrative concept, the theory of plate tectonics (formerly continental drift), rivals relativity and quantum mechanics in significance. Involving geophysical research in practically all fields of earth science, and paired with satellite surveys, plate tectonics constitutes a new inventory of natural resources and a scientific synthesis of the globe.

Geophysical exploration has, moreover, preserved its archetypal (original) forms, being international and corporate in composition, global in scale, and quantitative in data and being founded on the theoretical assumption of a steady state. It blends the styles of La Condamine and Humboldt, combining specific geophysical pursuits against a cosmic landscape. Yet the transformation is remarkable—the difference between Humboldt's surveying of sublime panoramas from the summit of the Andes, and the Earth Technology Resource Satellite (ETRS) radioing instrumental data to terrestrial computers.

Bibliography

Botting, Douglas. Humboldt and the Cosmos. New York: Harper and Row; London: Joseph, 1973.

Fraser, Ronald. Once Round the Sun: The Story of the International Geophysical Year. New York: Macmillan, 1957.

Glen, William. The Road to Jaramillo: Critical Years of the Revolution in Earth Sciences. Stanford, Calif. : Stanford University Press, 1982.

Goetzmann, William H. Exploration and Empire: The Explorer and the Scientist in the Winning of the American West. New York: Knopf, 1966; New York: Norton, 1978.

Hallam, Anthony. A Revolution in the Earth Sciences: From Continental Drift to Plate Tectonics. Oxford: Clarendon Press, 1973.

Oreskes, Naomi. The Rejection of Continental Drift: Theory and Method in American Earth Science. New York: Oxford University Press, 1999.

Sullivan, Walter. Continents in Motion: The New Earth Debate. New York: McGraw-Hill, 1974; American Institute of Physics. 1991.

—Steve Pyne/A. R.; F. B.